1. Field of the Invention
The present invention is related to a method and apparatus for logging while drilling oil well equipment, and, more particularly, to a method and apparatus for aligning surface and downhole measurements.
2. Description of the Related Art
The exploration for subsurface minerals requires techniques for determining the characteristics of geological formations. Many characteristics, such as the hydrocarbon volume, resistivity, porosity, lithology, and permeability of a formation, may be deduced from certain measurable quantities. Thus, the techniques for determining the measurable quantities must be accurate.
Conventional systems that measure characteristics of geological formations typically use logging while drilling equipment that transmit real-time data from a tool within a borehole to a surface device. Additional measurement data is typically collected from the same sensor or from another sensor in the tool and logged within the tool. After drilling is completed, the measurement data from both the tool and the surface device is typically merged to provide a more complete data set of the characteristics of the geological formation. Typically, the data is merged using a clock located in the tool and a clock located in the surface device, an index in the clock providing merge indicators for combining the data.
The merging of data often introduces errors into the data when events occur to the tool. For example, a tool in a borehole is exposed to very high pressures and inconsistent surroundings that may cause a clock within the tool to stop for a length of time. Thus, although data is recorded, the time period associated with the data may be time shifted. However, the exact length of the disruption is unknown. Existing techniques for determining the length of disruption typically require inefficient, time intensive analysis of the data to correct the errors, and these are generally unsuccessful.
What is needed, therefore, is a system and method for time aligning measurements stored in equipment such as logging while drilling with measurements received by surface devices.
Accordingly, a method for aligning measurements of a hostile environment includes synchronizing at least two clocks, wherein at least a first clock is in the hostile environment, measuring characteristics of the hostile environment, aligning time domain curves of the collected measurements, correlating the curves using one or more events recorded by each clock, and correcting the curves by adding one or more time offsets. A second clock may be located outside the hostile environment or within a same or similar hostile environment. The collecting the measurements may be taken by a first measurement tool in a first device near the first clock and in at least a second device coupled to a second clock. Further, the collecting the measurements may include transmitting the measurements to the second device via a slow communication channel such as a mud communication channel or a channel with compromised bandwidth. In an embodiment, the first device is within the measurement tool, the first device collecting the measurements via a faster communication channel. In another embodiment, the method includes correlating an event such as one or more of an accurately timed event by each clock and a discovered event recorded by each clock. The hostile environment may include one or more of a borehole environment, an environment with high environment pressures, and an environment wherein clock measurements are subject to error. The measurement tool may include one or more of a sonde, a logging while drilling tool, and a measurement while drilling tool.
In one embodiment, collecting data is performed in at least a first and a second measurement tool wherein the second measurement tool and the first measurement tool provide measurements, the measurements providing data with which to correct the aligning of the curves. The collecting the measurements may include plotting data from the first measurement tool and in a device coupled to a second clock, the plotting showing at least two measurements from a same sensor. The plotting may include determining a value for a characteristic with respect to a time value.
A system for aligning measurements of a hostile environment according to the present invention includes a measurement tool configured to measure characteristics of the hostile environment, the measurement tool including at least a first recording device, at least a first clock coupled to the first recording device, and at least a second clock synchronized to the first clock, the second clock coupled to at least a second recording device, the second recording device configured to receive measurements from the measurement tool. The measurements recorded by at least the first and second recording devices produce time domain curves that are aligned and correlated using one or more events recorded by at least the first and second clocks, and corrected by adding one or more time offsets.
One embodiment is directed to a method for aligning measurements of a hostile environment that includes receiving the measurements recorded by at least a first and a second recording device, wherein at least the first recording device is disposed in the hostile environment, aligning time domain curves of the collected measurements, correlating the curves using one or more events recorded by at least the first and second recording devices, and correcting the curves by adding one or more time offsets.
Another embodiment is directed to a computer program product including an input/output object operable to receive measurements recorded by at least a first and a second recording device, wherein at least the first recording device is disposed in a hostile environment, an alignment object coupled to the input/output object, the alignment object operable to align time domain curves of the collected measurements, a correlation object coupled to the alignment object, the correlation object operable to correlate the curves using one or more events recorded by at least the first and second clocks, and a correction object coupled to the correlation object, the correction object operable to correct the curves by adding one or more time offsets.